Para-xylene is a key intermediate in the synthesis of tetraphthalic acid. When methanol and toluene are passed over a heated bed of ZSM-5 zeolite, an equilibrium mixture of xylene isomers is formed. At temperatures of between 350 and 650° C., the equilibrium mixture is approximately 23% para-xylene, 51% meta-xylene and 26% ortho-xylene (W W Kaeding et al., Jn. of Catalysis, Vol. 67, 1981, pp 159-174). The selectivity to para-xylene can be enhanced by modifying the external and internal surfaces of the zeolite with oxides (K M Minachev et al., Proc. 9th Int. Congress of Catalysis, 1988, pp. 461-467).
U.S. Pat. No. 3,965,208 discloses an additional method for improving selectivity to para-xylene. A high temperature process (550-600° C.) is used at high space velocities. This reduces the contact time of the reactants with the catalyst. At such temperatures the catalyst becomes ‘coked’. This is beneficial as it has the effect of blocking external acid sites on the catalyst thus preventing isomerisation of the para-xylene. Selectivity is thus improved. However, ‘coking’ of the catalyst reduces the overall conversion of reactants to products. U.S. Pat. No. 3,965,208 teaches that increasing the reaction temperature improves the overall conversion efficiency, whereas lowering the temperature reduces conversion. The catalyst has to be periodically regenerated, or ‘de-coked’ to maintain an adequate balance between selectivity and conversion with a practicable process temperature. Under these conditions, increasing space velocity, or reducing contact time, is beneficial.
Another process for producing para-xylene by methylation of toluene is described in Japanese Laid-Open Patent Publication No. 144,324/1979, wherein a crystalline aluminosilicate catalyst is used and improving selectivity is observed at shorter contact times (see FIG. 2b). The document reports that whilst it is possible to increase the toluene conversion by prolonging the contact time, this reduces the para-xylene selectivity.
WO 98/14415 describes a process for the catalytic selective production of para-xylene using a catalyst derived from an optionally oxide modified ZSM-5. The catalyst is obtained by contacting the ZSM-5 with steam at temperatures of at least 950° C. The process is conducted at a temperature of from 500 to 700° C. and preferably from 500 to 600° C. pressures of from 100 KPa to 7000 KPa and a typical methanol utilisation of 50-70% wherein the catalyst accumulates coke as it catalyses the toluene and methylation reaction.
In addition to the toluene and methanol, WO 98/14415 contemplates adding hydrogen and/or water to the reaction mixture. It is clear from this that hydrogen and/or water is included in an effort to control excessive coke formation on the catalyst to extend the period between catalyst regenerations and/or to maintain conversion within acceptable limits
We have now discovered, very surprisingly, that by adding water to the reactant mixture and by reducing the contact time of the reaction mixture with the catalyst to below one second, it is possible to obtain industrially useful para-xylene selectivity with oxide modified ZSM-5 catalysts in conditions, such as temperature, wherein coking is limited or avoided.